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Most everyone in the world knows about owls. They are considered creatures of mystery or doom or good luck, depending on where you live. They are wonderful birds to discover during the day, like a baby
Northern Saw-whet Owl
at a nest hole (left) or a huge
Great Gray Owl
hunting near dusk in Yosemite National Park (below). But they are primarily nocturnal birds, hunting in the dark by using exceptional hearing and eyes that gather light much better than our own. Accordingly, our usual experience with them is after dark, like this
Tropical Screech-Owl
(above).

Since owls are night birds, they are difficult to see, but their characteristic songs and calls can often be easily heard. I enjoy owls but admit to not being much of an "owler." There are birders for whom owls are the ultimate challenge, and some who are good at imitations and whistles, but I'm not one of them. Perhaps this explains why my percentage of the world's owls seen is lower than most other families.

I prefer my owls in day roosts, like this
Long-eared Owl
at Pt. Lobos (far right). It was there a couple of weeks and then one day the roost was abandoned. Finding day roosts of owls is often pure serendipity, although sometimes one is clued in by pellets and white-wash on the ground below the roosts.

One of the least known owls I've seen was found at a day roost in eastern Gabon: this
Fraser's Eagle-Owl
(near right). There are few photographs of this enigmatic species of the Congo Basin rainforest.

And yet, maybe because it is difficult to see owls, each one is special, and encounters are memorable.

Marks et al. (1999) divided owls into three subfamilies [Striginae, Surniinae, Asioninae], and two of those have additional tribes, together comprising 7 subgroups of owls: scops-owls & screech-owls, eagle-owls, wood-owls, pygmy-owls & owls, hawk-owls (or boobooks), saw-whet owls, and eared owls. The latter, "eared owls," are just 9 species (including Long-eared Owl); many eagle-owls and screech-owls also have ear tufts. This is the classical views of owl taxonomy (e.g., Amadon & Bull 1988).

The first five photos have illustrated 5 of those 7 groups, leaving a need to illustrate an owlet — here's an
Asian Barred-Owlet
sitting on a roadside power line in China (left) — and a boobook— let's use
Ochre-bellied Boobook
of Sulawesi, which local guides located at a day roost (below).

New molecular evidence (e.g. Wink et al. 2008, Fuchs et al. 2008) has readjusted these lines a bit. There are still three subfamilies, but Ninoxinae (boobooks) takes the place of Asioninae. Those "eared owl" (like Long-eared Owl) are now just a tribe in a broader Striginae.

The 'new' Striginae now has two-thirds of the world's owls, including the scops-owl, screech-owls, eagle-owls, wood-owls, and the down-graded "eared owls." At least 50 species in the subfamily are in genus
Otus
— the scops-owls. Scops-owls are small owls of the Old World, and many have short, inconspicuous ear-tufts, as do
Mantanani Scops-Owl
(near right by B.T. Matheson), a small island specialist in the Sulu Sea, and
Palawan Scops-Owl
(far right), endemic to its namesake island in the Philippines. Scops-owls range from abundant species to some of the world's rarest (e.g., Seychelles Scops-Owl
O. insularis
, and Anjouan Scops-Owl
O. capnodes
in the Comoro Is., both listed as critically endangered). These are almost entirely nocturnal birds and day-roosts are changed from day to day. Vocalizations are often the most important isolating character in these species, and sometimes essential for identification. There is often more variation in plumage within a species — some of which may have distinctive gray, red, or brown morphs — than there is between species.

Likewise, in the New World, the screech-owls (genus
Megascops
), often have distinct color morphs within species that are more dramatic than variation between species. Here is a gray-morph
Eastern Screech-Owl
(below left, in a nice photo by Greg W. Lasley) and a red-morph
Tropical Screech-Owl
(below right). Both of these species have gray and red morphs (and even brown-gray morphs), and both of these species have pale green bills with yellow tips (while, for example, Western Screech-Owl
M. kennicotti
has a blackish bill). Separating screech-owls is much easier on vocalizations and (often) locality than it is on plumage characters. With ~50
Otus
owls in the Old World and ~25 screech-owls and relations in the New World, together these small owls comprise over a third of the world's owls.

Another 24 species are eagle-owls and relatives. These include such familiar birds as
Great Horned Owl
(right) in the New World. Surprisingly, though, the sister species to Great Horned Owl (Wink et al. 2008) is the circumpolar
Snowy Owl
(above). Snowy Owl may be the most widely migratory owl in the world, as it is now thought that most of the population moves south in winter (Marks et al. 1999), and of course it is famous for the irregular mass incursions southwards when the lemming prey base collapses. It is at such times that birds may reach the southern United States. In California, major incursions — involving dozens of individuals and sometimes even more —occurred in 1895, 1916-17, and 1973-74. At least 39 appeared in that latter winter, including the young owl above in northwestern California (nice shot by David Rudholm).

Great Horned Owl
must surely occupy the widest range of habitats of any owl, "from the limits of treeline in the north Nearctic south to the southern parts of South America" (Marks et al. 1999). It is found mostly in forested regions but "can also be met with in grassland, scrubland and entirely treeless habitats ... because it will accept a considerably broad spectrum of nesting sites, including cliff cavities and suitable ledges on rock faces and old buildings." It also does not shun populated areas; indeed, this youngster (right) roosted for several weeks in my front yard. A favored nest site in Monterey County is the re-use of old Red-tailed Hawk nests (Roberson & Tenney 1993), as this adult Great Horned is doing (below).

Among sought-after 'eagle-owls' are the three species of fish-owl (genus
Ketupa
) in Asia and the three species of fishing-owl (genus
Scotopelia
) in Africa. Locating a day roost is often the best strategy. Here, a
Brown Fish-Owl
roosts high in a large tree in India (below left), and
Pel's Fishing-Owl
(below center) is hidden among thick foliage in the Okavango Delta of Botswana.

I also still remember the
Buffy Fish-Owl
(just below) that hunted over the tennis courts at the Danum research station in lowland Borneo, where I caught it in a flash.

The Striginae also includes ~21 species of wood-owl, such as the lovely and dark-eyed Spotted Owl
Strix occidentalis
of virgin forests in western North America. Locally in Monterey County we thrill to its deep hoots in remote redwood canyons. There are 11 'eared-owls,' including Long-eared Owl
Asio otus
of the Holarctic, shown at the top of this page, and represented in Africa by two additional
Asio
and two species of white-faced owl (genus
Ptilopsis
).
Southern White-faced Owl
(left) is shown here, from an oasis in the barren grasslands of Etosha NP, Namibia.

Rounding out the subfamily are four spectacular owls in the Neotropics. Three are in the genus
Pulsatrix
, the most widespread of which is the huge Spectacled Owl
P. perspicillata
, with a range from s. Mexico to n. Argentina. An owl of tropical rainforests and backwater bayous, it descending, accelerating sequence of "pup-pup-pup-pup" calls can remind one of machine-gun fire. Much more scarce and local are Band-bellied Owl
P. melanota
of Andean foothills, and the striking
Tawny-browed Owl
(below left) in remnant patches of forest in the hills of southeastern Brazil. Perhaps the most impressive of all is
Crested Owl
(in monotypic genus
Lophostrix
), a scarce species of forests from Mexico to the Amazon Basin. It is a highly distinctive owl with its long white ear tufts and deep tawny facial disk. Ed Harper photographed this pair (below right) at a day roost in Ecuador.

There are ~40 species in subfamily Surniinae, and most of them are owlets and pygmy-owls. A widespread Neotropical species, perhaps the most familiar of them all, is
Ferruginous Pygmy-Owl
(above). The subfamily also include such interesting species as Elf Owl
Micrathene whitneyi
, the saw-whet and boreal owls (genus
Aegolius
), and the recently-discovered Long-whiskered Owlet
Xenoglaux loweryi
on northern Peru. Surprisingly, though, the largest member is the subfamily is
Northern Hawk-Owl
(right), and the name of the subfamily [Surniinae] is derived from its Latin name
Surnia ulula
. Despite its English name of "hawk-owl," this northern taiga-forest diurnal owl is not related to the "hawk-owls" in subfamily Ninoxinae. It is simply a historical accident that this Holarctic owl has a similar name to the tropical "hawk-owls" — and this is a reason to change the names of most
Ninox
owls to boobooks (more on that below).

One study of the hunting technique of Northern Hawk-Owl in Norway found that this diurnal owl searches for small mammals from elevated perches (like the top of the tree shown here), and that on average the owl visited five perches between each attack. The success rate of these attacks were 36% (Marks et al. 199).

It is duly noted that most pygmy-owls and owlets are also diurnal, or at least crepuscular, and often hunt in the light. In this the entire subfamily is quite different that the other two subfamilies of the family Strigidae. Many of the pygmy-owls and owlets, though, are bird-eaters, taking many species of small passerines, as well as lizards, grasshoppers, and larger insects.

An owl often seen during the day is
Burrowing Owl
(left, right & below). A bird of open country in the New World, males can often be seen 'on guard' in their territory. In the Brazilian chapada, termite mounds make excellent perches (left). Pairs breed in burrows, often dug by squirrels. As shown by this pair (below), it is the male that stands guard, as his plumage is washed-out by the summer sun, compared to her unbleached feathering.

The bulk of the Surniinae are ~ 26 species of pygmy-owl and owlets in genus
Glaucidium
. Some 17 are in the New World and nine in the Old World, although some may soon be assigned to separate genera. Many pygmy-owls are diurnal; indeed, some have evolved "false eye-spots" on the nape to make it appear they are looking both ways at once.
Northern Pygmy-Owl
(left, in a classic portrait by Gary Woods) is a species of western North America that is adapted to a variety of wooded habitats. It, like other pygmy-owls, is a curious bird and can often be 'called in' to the observer by those who can imitate their hollow high-pitched hoots. Those calls also attract many small birds, which will mob a real pygmy-owl in hopes of driving it from the vicinity.

Pearl-spotted Owlet
(right) of sub-Saharan Africa is an example of an Old World
Glaucidium
. It prefers open woodlands such as the African savanna.

Genus
Athene
is another interesting group, currently composed of both New World (i.e., Burrowing Owl) and Old World species. Whether this arrangement is permanent will require more study. Old World species include the very widespread
Little Owl
(below left, a photo from China) — a species of open country (and thus not unlike Burrowing Owl in habitat) — and
Spotted Owlet
of south Asia (below right, photo'd in India). Forest Owlet
A. blewitti
is often considered closely related. It was named in 1872 in India but was not seen for over a century until rediscovered in 1997, and has since been the subject of intensive surveys.

Many of these small owls depend on old woodpecker or barbet holes (depending on the continent) for nesting. Northern Pygmy-Owl and
Northern Saw-whet Owl
(left) lay their eggs on the bare floor on an old woodpecker hole. The youngsters are fed until they grow to adult size. Some, like Northern Saw-whet Owl, have a distinctive juvenal plumage. These two (left) fledged within two days of this photo, leaving behind the nest forever. Within a year, this dead tree collapsed. Thus the search of new holes, suitable for nesting, is an ever-present problem.

The third and final subfamily is the Ninoxinae — the boobooks. The one featured above, from New Zealand, has the distinct English name
Morepork
, a rendition of its onomatopoeic two-note call. As a group, though, the names of these small owls provide an example of people divided by a common language. Most British, or British-trained, ornithologists call these owls "boobooks;" but some American ornithologists working in the Old World tropics call them "hawk-owls" [they are not, however, remotely related to Northern Hawk-Owl of the high Holarctic which, as discussed above, is basically a very big pygmy-owl; it should simply become "Hawk Owl."]. The literature was pretty much split half-and-half on English names but recently the Clements world checklist (and eBird) adopted the name "boobook" for nearly the
Ninox
owls [excluding only a few Australian species with long-established names], and hopefully this will gain wide acceptance.

Consistent with English names like Morepork or Boobook, vocalizations are the key to these great little birds. They are usually small to medium-sized owls, and quite nocturnal. Nearly all of the nearly 30 species in the subfamily are within the single genus
Ninox
. These hunters perch horizontally, like Accipiters and many other hawks, and not upright like the typical Striginae owl. While most are smallish, Powerful Owl
N. strenua
of eastern Australia is quite large.

Another rather unique characteristic of some boobooks is their aggressive reaction to presumed rivals. Consider this
Mindoro Boobook
(right, in a very cool photo by Blake Matheson). This is a species — endemic to Mindoro Island of the Philippines — about which almost nothing is known. Note the half-open wings and aggressive stance as it stares down its rival — in this case, a tape-recording of its own call. This same responsive stance, with half-open wings, has been a feature of other
Ninox
owls called in with tapes at night.

Look at the dark
Ninox
shown below — it has that same stance, and this may be characteristic of the genus. This particular owl (below) is currently "an owl with no name." It has not yet been formally described to science, although photos and information about calls and habitat have been published (Madika et al. 2011), and its proposed English name might be "
White-spotted
Boobook
." It appears to be endemic to the cloud forests of central Sulawesi in Lore Lindu NP. Based on vocalizations, it is related to the recently described Cinnabar Boobook
N. ios
of northern Sulawesi (Rasmussen 1999), but that owl is bright cinnamon in color. I understand that specimens of the undescribed
Ninox
were collected in summer 2012 (
fide
Pamela Rasmussen), so we now await a formal description and more information about the relationships of this new owl.

[Note — in all cases, we left the hawk-owls 'winning' the vocal war, having apparently chased us away]

Although there is a lot to say about about owls, and the literature is extensive, there is still much to learn. As we have just seen, new species are still being discovered. Neither Cinnabar nor White-spotted Boobook made it into the
Handbook of Birds of the World
(Marks et al. 1999), nor did Sangihe Scops-Owl
O. collari
from Sangihe Is., nor Togian Boobook
N. burhani
from the Togian Islands. Even
Speckled Boobook
(right), the common
Ninox
near populated areas on Sulawesi, is considered to be little-known. Marks et al. (1999) said, for example, that there "is almost no information on diet; one record of killing a fruit bat caught in a mist net." It is presumed that many
Ninox
are insect-eaters (e.g., Rasmussen 1999) but this is primarily conjecture.

Vocalizations are the key to owls. Early classics emphasizing calls include the work of Joe Marshall (1967, 1978), and Marshall with Ben King for
Otus
(in Amadon & Bull (1988). A useful cassette accompanied Marshall's (1978) work on Asian owls; for the New World, the tape by Hardy et al. (1990) was revolutionary. The newest family book by Koenig et al. (2008) comes with a double CD with voices of many species, and more are now available on-line. The emphasis on vocalizations has led to major re-appraisals of species limits in the owls.

A survey of the owls by Amadon & Bull (1988), which included a reassessment of the genus
Otus
by Marshall & King, detailed only 162 species. By the time of Marks et al. (1999), new species and revised species limits had enlarged the total to 189 species. Koenig et al. (1998) listed 206 species, some requiring further substantiation. That "further substantiation" continues to be published. As I'm wrapping up this new version of the old web page, Dantas et al. (2015) was published, suggesting further splits in New World screech-owls and giving indication that other taxa require further research. The number of species of owls is increasing annually through research and the discovery of new species. The world list is at or approaching 215 species and growing. There are more different owls out there than had been anticipated, and there is still a lot to learn about most of them.

I don't own this book and haven't personally reviewed it. At this point in time, the
Handbook of the Birds of the World
account (Marks et al. 1999) seems to suffice. But I quote some reviews encountered on the internet: 'You will find this an excellent reference book, the most comprehensive on owls available;'
British Birds
(May 2009). 'Immerse yourself in its pages on a March evening, when spring is beckoning and local owls are starting to hoot temptingly outside the window;' BBC Wildlife (March 2009) 'This is, at this moment, the world's largest book of owls of the world;' Kaulbrief (September 2009). 'This is exactly what we have come to expect in a Helm Identification Guide: a book that is great for beginners, like me, and experts;' Peregrine (spring 2009). As I say, I've not seen this book but it comes well-recommended. I understand that it comes with a double CD with voices of many species.

There is even a more recent book — Heimo Mikkola. 2013.
Owls of the World: A Photographic Guide
. Firefly Books, Richmond Hill, Ontario, Canada — that is likely to be even more up to date. I have not seen it.

This is not a successful book for those interested in owl identification, taxonomy, distribution, or vocalizations (i.e., birders). But it doesn't really aim at that audience -- as one can tell from its title -- and its text may fulfill that goal for a general audience. It just happens that the book attempts to have illustrations of all the world's owls -- some nice photos intermixed with some marginal art and some zoo shots -- and range maps. There apparently is a new edition that I've not seen. Because there was no overshadowing world book, there are many books that cover bits of the world. Some examples are Voous' (1989)
Owls of the Northern Hemisphere
-- which does have some very fine artwork, including field sketches, by Ad Cameron, and a good solid text on biology -- and Karalus & Eckert's (1974)
The Owls of North America
-- which is similar is style but has a sparser text and tries (not always successfully) to illustrate by paintings all the races of North American owls. I am aware of similar books on South Africa, or Australia, etc. All these are "coffee-table" books, but many other types of book exist.